Plasma arc torches are widely used for workpiece processing, e.g., the cutting, piercing, and/or marking of metallic materials (e.g., elemental metals, metal alloys. etc.). A plasma arc torch generally includes an electrode mounted within a body of the torch (e.g., a torch body), a nozzle having a plasma exit portion (sometimes called an exit orifice, exit bore, or exit port) also mounted within the torch body, electrical connections, fluid passageways for cooling fluids, shielding fluids, and arc control fluids, a swirl ring to control fluid flow patterns in a plasma chamber formed between the electrode and nozzle, and a power supply. The torch produces a plasma arc, which is a constricted ionized jet of a plasma gas with high temperature and high momentum (e.g., an ionized plasma gas flow stream). Gases used in the plasma arc torch can be non-oxidizing (e.g., argon or nitrogen) or oxidizing (e.g., oxygen or ambient air).
In operation, a pilot arc is first generated between the electrode (e.g., cathode) and the nozzle (e.g., anode). Generation of the pilot arc may be by means of a high frequency, high voltage signal coupled to a DC power supply and the plasma arc torch, or any of a variety of contact starting methods. In some configurations, a shield is mounted to the torch body to prevent metal that is spattered from the workpiece (sometimes referred to as slag) during processing from accumulating on torch parts (e.g., the nozzle or the electrode). Generally, the shield includes a shield exit portion (also called a shield orifice) that permits the plasma jet to pass therethrough. The shield can be mounted co-axially with respect to the nozzle such that the plasma exit portion is aligned with the shield exit portion.
To achieve high plasma cutting speeds with good cut quality, a nozzle design that facilitates high current density is generally required. High current density has been achieved in plasma torches by reducing the size (e.g. diameter) of the nozzle orifice bore and/or extending the length of the nozzle bore to constrict the flow area of the plasma arc to increase the energy density (amps/in2) of the plasma arc. Nozzles with relatively high current densities and/or relatively high length/diameter ratios encounter relatively higher heat fluxes inside the nozzle bore from the plasma arc. The higher heat flux can lead to overheating of the nozzle, oxidation erosion of the nozzle material (e.g., copper), or local melting of the nozzle material. Higher heat flux can also lead to double arcing where the nozzle is eroded by series arcing between the nozzle, an electrode and a workpiece. Damage to the nozzle from overheating and double-arcing can cause the plasma jet or arc to widen and/or diverge due to less constriction by the nozzle shape, resulting in defects in cut quality such as increased angle variation, wide kerf, and excessive dross.
Generally, the erosion rate at a nozzle orifice is affected by the cooling efficiency at the nozzle orifice. Efficient cooling at this location can help to maintain a relatively low temperature, which leads to a lower erosion rate. For water-cooled nozzles used in high current plasma torches, cooling can include thermal conduction through solid metal or thermal convection through a coolant at or passed over the exterior surface of the nozzle. In some designs, additional cooling can be provided using shielding gas on an external surface of nozzle, for example, as is accomplished with the HPR™ torch designs of Hypertherm, Inc. of Hanover, N.H. and torches sold by ESAB of Florence, S.C., both of which employ relatively high electrical currents. Generally, cooling a nozzle with a gas flow is weaker or less effective than cooling the nozzle with water or another liquid.
The heat that is absorbed by the tip of a nozzle near the exit orifice (mostly at the orifice wall) needs to be removed. Heat removal is accomplished by both thermal conduction, and by water convection at the upper part of nozzle and/or thermal convection with the shielding gas (if the nozzle cooling by gas is not negligible). In most cases the gas cooling function is negligible compared with water cooling, so all or most of the total heat transfer through the nozzle is performed by the water cooling. In these situations, the heat transfer rate from the nozzle orifice to the cooling water is controlled mainly by the temperature gradient at the nozzle tip (e.g., near the nozzle exit orifice), the temperature difference between the solid/liquid interface (e.g., between the nozzle material, such as copper and the cooling liquid such as water) and initial cooling water, and the convection heat-transfer coefficient. This can be described by two governing relations, namely the thermal conduction equation (Equation 1 below) and the thermal convection equation (Equation 2 below).
                              Q                      A            Orifice                          =                              -            k                    ⁢                                                    T                                  S                  /                  L                                            -                              T                Orifice                                                    Δ              ⁢                                                          ⁢              X                                                          (                  Equation          ⁢                                          ⁢          1                )            where:
Q is the total heat flux at the nozzle orifice, assuming heat is transferred only through the nozzle orifice wall;
AOrifice, is the surface area of the nozzle orifice bore;
k, is the thermal conductivity of the nozzle material;
TS/L, is the temperature at the interface of the nozzle and the cooling fluid (e.g., water);
TOrifice, is the temperature at the nozzle orifice bore, assuming a uniform temperature; and
ΔX, is the effective distance from nozzle orifice to solid liquid interface.
                              Q                      A                          S              /              L                                      =                  h          ⁡                      (                                          T                                  S                  /                  L                                            -                              T                0                                      )                                              (                  Equation          ⁢                                          ⁢          2                )            where:
AS/L is the area of the interface of the nozzle and the cooling fluid;
h, is the convection heat-transfer coefficient of the nozzle material;
TS/L, is the temperature at the interface of the nozzle and the cooling fluid; and
T0, is the initial temperature of the cooling fluid.
The conventional design approach for nozzle cooling is to bring the coolant material as close to the nozzle exit orifice as possible, for example, by reducing the distance therebetween, e.g., the ΔX term in Equation 1. However, reducing the distance between the coolant and the nozzle exit orifice can be limited by the ability of thermal convection to maintain the solid/liquid interface temperature at an acceptable level. There have been several approaches to make the coolant seal.
FIG. 1 depicts a partial cross-sectional view of a typical design of a system 100 for cooling a tip 105 of a nozzle 110. The system 100 is generally representative of the Proline 2200™, 200-amp nozzle sold by Kaliburn, Inc., of Charleston, S.C. The system 100 includes a nozzle 110 and a nozzle retaining cap 115 that is detachably mounted to a torch body (not shown) to secure the nozzle 110 relative to the torch body. A full cross-sectional view of the system 100 would include a mirror image of the components of the system 100 disposed symmetrically about the centerline or longitudinal axis A. The tip 105 defines an exit orifice portion 120 through which a plasma jet (not shown) exits the nozzle 110. The tip 105 also defines a recessed portion 125 into which a sealing component 130 (e.g., an o-ring) is disposed to form a fluid-tight seal between the nozzle 110 and the nozzle retaining cap 115. The tip 105 also defines a first shoulder portion 135 configured to mate with a corresponding flat portion 140 of the nozzle retaining cap 115 to form a metal-to-metal interface 143 therebetween. The interface 143 provides a heat conduction path between the nozzle 110 and the nozzle retaining cap 115. A rear portion 145 of the nozzle 110 cooperates with a corresponding rear portion 150 of the nozzle retaining cap 115 to form a chamber 155 through which a cooling fluid (not shown) flows.
The interface 143 secures the nozzle 110 to the torch and positions the nozzle 110 relative to the longitudinal axis A. The system 100 is generally representative of the PBS-75/PBS-76 nozzle sold by Kjellberg Elektroden und Maschinen GmbH, of Finsterwalde, Germany. Several drawbacks exist with respect to the system 100. For example, the configuration of the system 100 results in insufficient cooling of the nozzle tip 105. The recessed portion 125 overheats, resulting in overheating and/or burning of the sealing component 130. Failure of the sealing component can lead to failure to create a fluid-tight seal between the nozzle 110 and the nozzle retaining cap 115, resulting in leakage of the cooling fluid, premature failure of the nozzle 110 or nozzle retaining cap 115, and/or damage to other torch components not shown in FIG. 1A (e.g., the torch electrode or shield). Moreover, the recessed portion 125 results in less metal-to-metal contact between the nozzle 110 and nozzle retaining cap 115, which reduces the surface area of physical contact therebetween.
The system 100 also results in a “stagnation zone” 158 in the chamber 155 near the interface 143. The fluid flow in the “stagnation zone” 158 is slower relative to other portions of the chamber 155, resulting in reduced convective cooling near the “stagnation zone” 158. The “stagnation zone” 158 exhibits fluid flow resistance that hinders relatively cooler fluid from flowing into the “stagnation zone” 158, which reduces the convective cooling effect of the fluid.
FIG. 1B is a partial cross-sectional view of a design of a second or alternative system 160 for cooling the tip 105′ of a nozzle 110′. The system 160 includes the nozzle 110′ and the nozzle retaining cap 115′. A full cross-sectional view of the system 160 would include a mirror image of the components of the system 160 disposed symmetrically about the centerline or longitudinal axis A. The tip 105′ defines an exit orifice portion 120′ through which a plasma jet (not shown) exits the nozzle 110′. The tip 105′ includes a sealing portion 165 in physical contact with a corresponding portion 170 of the nozzle retaining cap 115′ to form a metal-to-metal interface 175 therebetween. A rear portion 180 of the nozzle 110′ cooperates with a corresponding rear portion 185 of the nozzle retaining cap 115′ to form a chamber 155′ through which a cooling fluid (not shown) flows.
The interface 175 secures the nozzle 110′ to the torch and positions the nozzle 110′ relative to the longitudinal axis A. The interface 175 generally also acts as a fluid seal to hinder the cooling fluid flowing in the chamber 155′ from leaking. Several drawbacks exist with respect to the system 160. For example, difficulties exist in manufacturing or machining the nozzle 110′ and/or the nozzle retaining cap 115′ to achieve a fluid-tight seal at the metal-to-metal interface 175. As a result, the interface 175 tends to leak cooling fluid during operation (e.g., as the operating temperature of the nozzle 110′ (and tip 105′) increases. After a leak has developed between the nozzle 110′ and the nozzle retaining cap 115′, both generally must be replaced. Moreover, failure of the interface as a seal 175 can result in damage to the torch electrode (not shown) and a shield (not shown), which generally also require replacement.
Generally, the nozzle 110′ and the nozzle retaining cap 115′ are manufactured from different materials. For example, the nozzle 110′ is frequently made of copper or copper alloys, and the retaining cap 115′ commonly made of brass. The different materials have different coefficients of thermal expansion, which affects how quickly the nozzle 110′ and the nozzle retaining cap 115′ expand during heating (e.g., during torch operation) and contract (e.g., during cooling or thermal relaxation). The interface 175 tends to be sensitive to metal dust and/or the surface finish of the nozzle 110′ or the nozzle retaining cap 115′.